
I am not familiar with the change-of-variable technique that the question refers to. Does anyone have an idea what is meant and how one should go about doing it?

I am not familiar with the change-of-variable technique that the question refers to. Does anyone have an idea what is meant and how one should go about doing it?
the change-of-variables techniqueis meant the usual method involving Jacobians. However, the result that you are asked to prove is false unless the univariate standard normal random variables are independent random variables. See this answer for a great description of how two normal random variables can fail to have a bivariate normal distribution. – Dilip Sarwate Oct 21 '13 at 12:16